This invention relates to blends of phosphors that convert electromagnetic radiation in the near-ultraviolet (“near-UV”)-to-blue wavelength range to white light. More particularly, this invention relates to phosphor blends that convert electromagnetic radiation emitted by UV light-emitting devices to white light. This invention also relates to light sources using such phosphor blends.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color and luminosity of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or x rays (for example, scintillators in x-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. Therefore, in the quest for improving efficiency of white light sources, effort has been dedicated to finding a source of stimulating radiation that has wavelengths longer than that of UV radiation and phosphors that respond to those wavelengths. Recent advances in light-emitting diode (“LED”) technology have brought efficient LEDs emitting in the near UV-to-blue range. The term “LEDs” as used herein also includes laser diodes. While these lighting devices yield an emission spectrum that has a high color rendition index (CRI) value, the luminosity obtained from the device is lower than that desired. In certain applications, such as the instrument panel of an automobile, a high CRI value is desired, while the luminosity is not critical. It would be an advance to the technology of lighting to provide a range of phosphors that can be stimulated by the radiation emitted from near-UV/blue LED radiation sources to allow for flexibility in the use of phosphors for generating emitted light with a high luminosity at an acceptable CRI level. Such phosphors when combined with the emission from the near-UW/blue LEDs can provide luminous and efficient lighting devices that consume little power.
Therefore, there is a need to provide phosphor compositions that are excitable in the near UV-to-blue range and emit in the visible range such that they may be used flexibly to design light sources with a high luminosity.